Typical building fire alarm systems include a number of fire detectors positioned throughout a building. Signals from those detectors are monitored by a system controller, which, upon sensing an alarm condition, sounds audible alarms throughout the building. Flashing light strobes may also be positioned throughout the building to provide a visual alarm indication. A number of notification appliances comprising audible alarms and strobes, the audible alarms and strobes being generally referred to as notification devices, are typically connected across common power lines on a notification circuit.
A first polarity DC voltage may be applied across the notification circuit in a supervisory mode of operation. In this supervisory mode, rectifiers at the notification appliances are reverse biased so that the alarms are not energized, but current flows through the power lines at the notification circuit to an end-of-line resistor and back, allowing the condition of those lines to be monitored. Because notification circuits are supervised using an end-of-line resistor, the wires of the circuit must be a single continuous run with no branches and an end-of-line resistor across the wires at the end farthest from the system controller. With an alarm condition, the polarity of the voltage applied across the power lines is reversed to energize all notification appliances on the notification circuit.
U.S. Pat. No. 5,559,492 issued to Stewart et al. (hereinafter the '492 Stewart patent) operates according to the system described above. The '492 Stewart patent further discloses that the visual alarms, or strobes, may be synchronized to fire simultaneously resulting from power interruptions, also referred to as synchronization pulses, in the power lines. Additional timing lines for synchronizing the strobes are not required because the synchronizing signals are applied through the existing common power lines.
Other alarm systems have controlled the function of the audible and visual alarms by interrupting the power signal to the alarms in a predetermined pattern as control signals over the common power lines or by communicating during the synchronization interruption of power. The audible and visual alarms operate their respective loads responsive to the control signal received.